So the easy answer is definitely just don't do it.
If you wanted to try, though, here's what I'd do:
1. Freeze activity on the new server
2. Systematically merge the user, repository, review group, etc. tables.
This would require mapping any duplicates to their counterparts on the new
server,
So, I suspect this is going to be a tricky situation to solve:
I've been hosting a Review Board instance on Red Hat's public OpenShift
for quite a while while we worked on getting a supported version hosted
in the Fedora Hosted infrastructure. There are now over 1500 reviews in
this OpenShift
